In this application, we apply for a competitive renewal of the grant that funds the Vitamin D and Omega-3 Trial to prevent and treat Diabetic Kidney Disease (VITAL-DKD). During our initial period of funding, we launched VITAL-DKD as an ancillary study to the parent Vitamin D and Omega-3 Trial (VITAL). Specifically, to the large (N=25,875) and simple parent VITAL trial, we added kidney outcomes and accompanying clinical data to a subset of 1,322 VITAL participants with prevalent type 2 diabetes to test the effects of vitamin D3 and omega-3 fatty acids on the clinical manifestations of DKD. In this competitive renewal application, we propose to extend follow-up of our VITAL DKD ancillary study from 2 to 5 years, add measurements to stored specimens to more comprehensively evaluate treatment effects, and assess intermediate cardiovascular outcomes that complement our primary renal outcome. Our approach builds on our recruited study population and completed measurements and leverages the infrastructure of the parent VITAL trial, which is funded by the NIH to provide study treatments and basic data acquisition through 2017. We have recruited a diverse study population with type 2 diabetes that has or is at risk of DKD and is maintaining excellent adherence to study medications. To extend our study, our experienced study team will apply methods developed and successfully implemented during our initial funding period. Preliminary data demonstrate high retention and adherence rates and successful laboratory protocols. We anticipate that the additional studies proposed herein, when added to those of the initial funding period, will generate a comprehensive evaluation of the long-term effects of vitamin D3 and omega-3 fatty acids on DKD.